1. Field of Use
The invention of the present application relates to a substrate holder used in a plasma processing system that uses plasma to perform a desired process on the surface of a substrate.
2. Description of Related Art
The use of plasma to perform a desired process on a substrate is frequently encountered in the production of various semiconductor integrated circuits such as DRAMs and in the production of liquid crystal displays. For example, when forming microcircuits on a substrate, plasma is used to etch the substrate by plasma etching in etching stages where a resist pattern is used as a mask. In the fabrication of various conductive films and insulating films, it has become practical to use plasma CVD (chemical vapor deposition) methods which employ vapor-phase reactions in a plasma.
In this sort of plasma processing system, a substrate holder is needed to hold the substrate that is processed with plasma inside the process chamber. FIG. 4 is a schematic cross-sectional front view showing the structure of a conventional substrate holder for a plasma processing system.
The substrate holder for a plasma processing system (abbreviated to substrate holder in the following) functions as a "mount" that holds and supports the substrate, and as a "temperature regulation means" that regulates the temperature of the substrate. The substrate holder shown in FIG. 4 has a roughly cylindrical holder main body 1 which functions as a "mount" and a substrate temperature regulation mechanism 5 which regulates the temperature by causing a temperature medium to flow through the interior of the holder main body 1.
The holder main body 1 comprises a substrate holding plate 2 which is the member on which the substrate 10 is supported, a main block 3 on which substrate holding plate 2 is mounted, and a contact sheet material 4 which is interspersed between the substrate holding plate 2 and the main block 3.
The upper surface of the substrate holding plate 2 constitutes a substrate holding surface 20. This substrate holding surface 20 is circular, and slightly smaller than the diameter of the substrate 10. Also, a brim part 21 extends out from a position slightly below the substrate holding surface 20.
The main block 3 is made of a metal such as aluminum or stainless steel. The main block 3 is a member which has a roughly cylindrical shape, with the upper parts having a slightly larger diameter.
The contact sheet material 4 is a member for improving the thermal contact between the main block 3 and the substrate holding plate 2. The contact sheet material 4 is formed from a metal such as indium, and is embedded in the gap between the main block 3 and the substrate holding plate 2 so as to improve the thermal contact between the two.
The periphery of the holder main body 1 is covered with an insulating block 11. The insulating block 11 is provided to prevent the holder main body 1 from being damaged by the plasma, and is formed of a heat-resistant insulator such as a fluoroplastic.
The substrate temperature regulation mechanism 5 regulates the temperature by introducing a temperature control medium (abbreviated to temperature medium in this specification) into the main block 3. A temperature medium cavity 51 into which the temperature medium is introduced is formed inside the main block 3. The temperature medium cavity 51 is an annular space with a slightly larger outer diameter than the substrate 10. A temperature medium supply path 52, whereby the temperature medium is supplied to this temperature medium cavity 51, and a temperature medium discharge path 53, whereby the temperature medium is discharged from temperature medium cavity 51, are formed in this temperature medium cavity 51.
The substrate temperature regulation mechanism 51 consists primarily of a temperature medium supply pipe 54 connected to the temperature medium supply path 52, a temperature medium discharge pipe 55 connected to the temperature medium discharge path 53, and a circulator 56, which is provided so as to link the temperature medium supply pipe 54 with the temperature medium discharge pipe 55. A circulator 56 has a temperature regulating part, such as a thermostat, which maintains the temperature medium supplied from the temperature medium supply pipe 54 at a fixed temperature.
The holder main body 1 is provided with a temperature sensor (not illustrated), and the signal from this temperature sensor is fed back so as to maintain the temperature medium at a fixed temperature. Although the choice of temperature medium depends on the temperature being regulated, tap water is often used in typical cases.
To improve the precision of temperature regulation with the substrate temperature regulation mechanism 5, a contact improvement means which improves the planar contact of the substrate 10 against the substrate holding surface 20 is provided. This contact improvement means is formed by an electrostatic chucking mechanism 6 which causes the substrate 10 to be attracted electrostatically to the substrate holding surface 20, and a gas supply mechanism 7 which supplies a specific gas to the gap between the substrate 10 and the substrate holding surface 20.
The electrostatic chucking mechanism 6 primarily consists of a chucking electrode 61 embedded in the substrate holding plate 2, and a high-frequency power source 63 and/or a DC power source 64, which applies a specific voltage to the chucking electrode 61. The substrate holding plate 2 is formed of a dielectric such as alumina (Al203).
Specifically, a conductor 62 is embedded so as to reach from the chucking electrode 61 to the contact sheet material 4. The high-frequency power source and the DC power source are connected to the main block 3. Of these, the high-frequency power source 63 applies a self-bias voltage to the substrate 10 by the interaction between the plasma and the high-frequency.
Plasma P is supplied to the top side of the substrate 10. Here, when a high-frequency voltage is applied to the substrate 10 with the substrate holding plate 2, which is made of a dielectric, acting as a capacitor, charged particles in the plasma are periodically attracted to the substrate 10. Electrons, which have high mobility, are attracted to the substrate 10 in greater numbers than ions, and as a result, the potential of the substrate 10 is self-biased in the same way as if a negative DC voltage were superimposed on the high frequency.
Although electrostatic attraction of the substrate 10 can sometimes arise from the induction of static electricity in the surface of the substrate holding plate 2 by this self-bias voltage, the DC power source 64 applies a DC voltage to make this attraction more secure. The DC power source 64 is made to apply a fixed positive voltage to the chucking electrode 61. Since the surface of the substrate holding plate 2 is negatively biased and the chucking electrode 61 has a positive potential, a large potential difference occurs in the dielectric between the adhesion electrode 61 and the substrate holding surface 20, and the dielectric is thereby strongly polarized. As a result, a large static potential is induced in the substrate holding surface 20 and the substrate 10 is electrostatically attracted by the Coulomb force.
A force is also generated by the Johnson-Rahbeck effect between the substrate 10 and the substrate holding surface 20, and the substrate 10 is also attracted by this Johnson-Rahbeck force. The Johnson-Rahbeck force arises from the charge polarization that occurs due to a minute current flowing across the small gap between the substrate 10 and the substrate holding surface 20.
Which of these forces is dominant is determined by the volume resistivity of the dielectric from which the substrate holding plate 2 is formed. The Coulomb force constitutes a larger part of the attraction when the dielectric resistivity is large, and the Johnson-Rahbeck force constitutes a larger part of the attraction when the dielectric resistivity is small.
Meanwhile, as FIG. 4 shows, a gas supply path 71 is formed so as to pass through the main block 3 and the substrate holding plate 2. The gas supply mechanism 7 consists of a gas supply pipe 72 which is connected to a gas supply path 71, and a gas cylinder 73 which holds the gas that is supplied to the gas supply path 71 through the gas supply pipe 72.
The gas supply pipe 72 is provided with a mass-flow controller 73 whereby it is made to supply gas at the desired rate. The tip of the gas supply path 71 is made into an opening formed in the substrate holding surface 20, and the gas from the tip opening is supplied between the substrate 10 and the substrate holding surface 20. The supplied gas is a gas with high thermal conductivity, such as helium.
Neither the back surface of the substrate 10, nor the substrate holding surface 20, is a physically perfect flat surface, and a minute space is formed between the two. In this minute space, no heat is conveyed by direct conduction between the substrate 10 and the substrate holding surface 20. Since the substrate holder is situated in a vacuum in most cases, it is also difficult for heat to be conveyed by gas convection. The gas supplied by the gas supply mechanism 7 to the minute space between the substrate 10 and the substrate holding surface 20 solves this problem by mediating the exchange of heat between the two.
FIG. 5 is a front view showing the schematic configuration of a plasma processing system in which the substrate holder shown in FIG. 4 is mounted.
The plasma processing system shown in FIG. 5 primarily consists of a process chamber 81 equipped with a pump-down system 811, a plasma supply means 82 which supplies plasma P into process chamber 821, and a substrate holder 83 which holds the substrate 10 to be processed by the supplied plasma p.
This plasma processing system forms a helicon wave excited plasma. The plasma supply means 82 primarily consists of a dielectric vessel 821 which has an airtight connection to process chamber 81, a gas supply means 822 which supplies a process gas into the dielectric vessel 821, an electrical power supply means 823 which supplies high-frequency electrical power into the dielectric vessel 821 and thereby turns the gas into a plasma, and an electromagnet 824 which establishes a magnetic field inside the dielectric vessel 821.
The electrical power supply means 823 induces a circularly polarized high frequency wave (a helicon wave) into the dielectric vessel 821 via a loop antenna. This forms a high density plasma inside the dielectric vessel 821. The high density plasma diffuses into the process chamber 81 under the magnetic field established by the electromagnet 824, and is used to process substrate 10.
When performing plasma etching, a fluorocarbon gas such as carbon tetrafluoride (CF.sub.4) is used as the process gas. Fluoride ions and/or fluoride active species are produced in the plasma. These ions and/or active species arrive at the substrate 10, whereby the material (e.g., silicon oxide) on the surface of the substrate 10 is etched.
When depositing a thin film of amorphous silicon by plasma enhanced CVD, the plasma is formed by introducing a mixed gas of hydrogen gas and a silane gas such as monosilane (SiH.sub.4) as the process gas. Decomposition of the silane gas in the plasma is used to deposit a thin film of amorphous silicon on the surface of substrate 10.